1. Field
This disclosure relates to stereoscopy.
2. Description of the Related Art
Humans view the environment three-dimensionally using binocular vision. Binocular vision is both a visual system and an analytical system. Our brain perceives both distance and speed based, in part, on triangulating visual light information received by the retinas of our respective laterally separated, forward facing eyes. Since both eyes are forward facing, the fields of view of each of our eyes overlap, with each eye perceiving a slightly different perspective of the same area. As we focus on objects closer to our eyes, our eyes rotate towards each other. As we focus on objects afar, our eyes rotate towards a parallel view. The angle between the lines of sight of each eye is commonly termed the convergence angle. The convergence angle is higher when we view objects closer to our eyes and lower when viewing distant objects. The convergence angle may be essentially zero, indicating essentially parallel lines of sight, when we view objects at great distance.
Three dimensional imaging, also known as stereographic imaging, dates at least as far back as 1838. Historically, stereographic cameras commonly included two lenses spaced laterally apart a similar distance as an average human's eyes, approximately 65 mm. The effective distance of the lenses from each other is known as the interocular distance. The interocular distance has a strong effect on the apparent depth of a stereographic image. Increasing the interocular spacing increases the apparent depth of a stereographic image. Decreasing the interocular spacing has the effect of decreasing the apparent depth of a stereographic image.
The presentation of stereoscopic images is commonly achieved by providing a first image to be seen only by the left eye and a second image to be seen only by the right eye. Differences, or disparity, between the two images may provide an illusion of depth. Two images having disparity may be perceived as three-dimensional. Two images, or portions of two images, exhibiting excessive disparity may not be perceived as three-dimensional, but may simply be seen as two overlapping two-dimensional images. The amount of disparity that a viewer can accommodate, commonly called the disparity limit, varies among viewers. The disparity limit is also known to vary with image content, such as the size of an object, the proximity of objects within an image, the color of objects, and the rate of motion of objects within the image. The disparity limit, expressed as the angle between the lines of sight of the viewer's eyes, may be about 12-15 minutes of arc for typical stereoscopic images.
A variety of techniques, including polarization, filters, glasses, projectors, and shutters have been used to restrict each eye to viewing only the appropriate image. One approach to displaying stereographic images is to form the left-eye image on a viewing screen using light having a first polarization state and to form the right-eye image on the same viewing screen using light having a second polarization state orthogonal to the first polarization state. The images may then be viewed using glasses with polarizing lenses such that the left eye only receives light of the first polarization state and the right eye only receives light of the second polarization state. Stereoscopic displays of this type typically project the two polarized images onto a common projection screen. This technique has been used to present 3-D movies.
A second approach to displaying stereographic images is to form the left-eye and right-eye images alternately on a common viewing screen at a high rate. The images may then be viewed using shutter glasses that alternately occult either the right or left eye in synchronism with the alternating images.
Preparing a conventional, non-stereographic camera to record two-dimensional images typically involves setting an aperture or lens speed, a shutter or exposure speed, a lens focal length or zoom, and a lens focus distance. Both still picture and video cameras for consumer use commonly include a method for selecting preset operating modes that set at least some of these parameters to predetermined values. Common preset modes include portrait (typically sets a large aperture), sport (typically sets a fast shutter speed), and landscape (typically sets a small aperture). Users of stereographic cameras may need to set additional parameters, including an interocular distance and a convergence angle.
Throughout this description, elements appearing in schematic diagrams and block diagrams are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. Elements that have similar functions for either the left or right eyes are assigned the same reference designator with a suffix of either “L” or “R” to indicate left-eye or right-eye, respectively.